While known digital baseband imaging systems, such as of the ultrasonic imaging type, provide a number of features, their performance is limited by practical system considerations. One such limitation lies in the use of phase focusing. Fine focusing delay and dynamic focusing are provided by phase rotation of the real and imaginary parts of the baseband signal rather than by actual delay of the signal. This method provides an exact solution for narrow-bandwidth signals at the central frequency, but generates errors of increasing magnitude as the bandwidth of the signal is increased.
In digital baseband imaging systems having two summation channels, the radiofrequency signals are converted into digital samples by an analog-to-digital converter. These digital samples then undergo a series of demodulations and filtering to generate a complex signal having in-phase and quadrature components. These operations are carried out separately in each receive channel. The echo signals reflected from a particular sample volume arrive at the various transducers at different times. A reception beam from that sample volume is formed by applying precision time delays to the signals received by respective receive channels from the transducer array. Each of these signals is delayed by an amount necessary to form a beam in the given desired direction. This beamforming process comprises two parts: the steering function and the focusing function. The steering function is realized by providing the time delay necessary to steer the beam in a given direction .theta. with respect to a line normal to the face plane of the transducer array. The focusing function is realized by providing a time-dependent time delay necessary to maintain accurate focus during propagation of the imaging energy pulse through the sample. These delays enable the various signals reflected from each point (R, .theta.) to be summed into one coherent summation signal.
The quantization of the beam-steering time delay, however, is limited to the discrete time steps of the analog-to-digital converter. In some known systems, the analog-to-digital converters operate with a clock frequency of 40 MHz. The time delays are therefore quantized to steps of 25 nsec. This forms a practical limitation of such systems. If selecting the cycle of a common reference clock were the only method for providing time delay, the resulting image would be poor. However, as already mentioned, these systems further employ phase delay to remove the quantization in time restrictions of the analog-to-digital converters and to improve the overall image quality.
Phase rotation of the demodulated and filtered signals is accomplished by vector rotation of the in-phase and quadrature components of the complex signal. This rotation is fixed in time to provide the basic steering or beamforming direction and is changed in time to provide dynamic focusing of the received beam. This method produces an exact solution for narrow-bandwidth signals at the central frequency, but generates errors of increasing magnitude as the bandwidth of the signal is increased.
In prior art digital baseband imaging systems having two summation channels, two separate phase rotations are performed to provide two separate beams from a single firing of the transducers. The two formed beams are directed relative to each other at different angles. This feature is commonly referred to as two-for-one beamforming. Each channel has its separate schedule of both the initial and the time-dependent phase changes, but both beams utilize the same time-delayed, demodulated and filtered signal. The rotated output signals are separately summed by the left and right summation channels and sent to the system midprocessor for further processing and display.